Control of the solar energy coming into windows is important in maintaining comfortable indoor conditions in warm climates. Window materials have been developed also to reduce glare. Solar control has been achieved commonly by adding absorbing colorant material to the glass. Tinting of glass in this way has disadvantages in production, however, since a long time may be required to change shades. More recently, reflecting and absorbing films have been applied to clear glass, to achieve solar control. Reflection of unwanted radiation is more efficient than absorption, since reflection eliminates the radiation completely, whereas part of the absorbed heat is eventually carried into the building.
Processes for the application of reflective and absorptive solar control films are well-known in the area of glassmaking. For example, films of metals such as chromium or nickel are evaporated or sputtered onto glass in vacuum, using equipment which is commercially available and well-known in the art. While good quality reflective and absorptive films are produced by vacuum methods, the cost can be rather high. Mixtures of metal oxides, such as chromium oxide, cobalt oxide and iron oxide, can be deposited by spray pyrolysis, as described for example in U.S. Pat. No. 3,652,246. Similar films have been made by chemical vapor deposition, as described for example in U.S. Pat. No. 3,850,679 and by pyrolysis of finely powdered materials as described in U.S. Pat. No. 4,325,988. These films are not as reflective as the vacuum-deposited metals, but they can be prooduced more cheaply. They do require materials such as cobalt and chromium, which have limited sources of supply, and must be imported into the U.S. Also, chromium and nickel are suspected of causing cancer, so the safety of such coated products for widespread use may be questioned.
It has also been proposed in U.S. Pat. No. 3,885,855 to produce solar control films by reactive sputtering of the nitrides, carbides or borides of the metals titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, or tungsten. While effective optical properties were known for some of these materials, any largescale production of architectural glass by the vacuum electrical method of reactive sputtering would be rather expensive.
The machine tool industry has utilized hard, relatively thick, opaque, wear-resistant coatings of titanium nitride. These coatings are formed at very high temperatures, say 1000.degree. C., with a nitrogen, hydrogen and titanium tetrachloride reaction mixture. However, Japanese Pat. No. 74-83679 and Swedish Pat. No. 397,370 have disclosed such wear-resistant coatings, all of which are functionally opaque and at least about three microns thick, to have been formed from the reaction of ammonia and titanium tetrachloride at temperatures in the 550.degree. C. range.
U.S. Pat. No. 4,310,567 describes formation of nitride coating, but no process is disclosed which is capable of providing thin transparent films for solar applications. U.S. Pat. No. 4,196,233 to Bitzer also describes a nitride coating process.